Holographic lighting in Cycles
Yes, this is possible using volumetrics:

Method 1:
Using a cube to scatter light emitted from a spot lamp.

Create a cube and give it a volume scatter material:


Add a spot lamp:



Method 2:
You could also fake this by modeling the cone, then using a volumetric gradient for the falloff:

It won't be as physically accurate or as flexible, but it will be less noisy:


Example .blend (for both methods)